1. Field of the Invention
This invention generally relates to the estimation of position and orientation of an object with respect to a local or a global coordinate system. In particular, the invention relates to the method and apparatus that provides estimation and tracking of the position and orientation. The method and apparatus that can be used in vehicles, such as in mobile robots.
2. Description of the Related Art
Position estimation is a topic of interest for a wide variety of application areas ranging from autonomous mobile robots, ubiquitous computing, mobile devices, tracking of assets, tracking of people, position tracking of customers in a store, tracking of pets, position of nodes in ad hoc wireless networks, position tracking of vehicles, and position tracking of mobile devices such as cell phones, personal digital assistants, and the like.
Robots are becoming more and more commonplace in society. It will be understood that these robots can be embodied in a variety of forms, such as in automated floor care products such as vacuum cleaners. A variety of applications can be found for mobile robots, such as, but not limited to, entertainment applications such as toy robots, healthcare applications such as elderly care robots, patrolling and security applications, telepresence robots, cleaning applications such as floor cleaning robots, utility applications in environments that are unfriendly to humans such as space, deep water, cold temperature, radiation, chemical exposure, biohazards, etc., dangerous tasks such as defusing of potential explosives, operation in confined spaces such as collapsed buildings, and the performance of menial tasks such as cleaning. Mobile robots, robots that can move from one location to another, often use knowledge of their position relative to their environment.
Localization techniques refer to processes by which a robot determines its position and orientation relative to a reference coordinate system. The reference coordinate system can be either local (for example, relative to an object of interest) or global. Position estimation can include estimation of any quantity that is related to at least some of an object's six degrees of freedom of in three dimensions (3-D). These six degrees of freedom can be described as the object's (x,y,z) position and its angles of rotation around each axis of a 3-D coordinate system, which angles are denoted α, β, and θ and respectively termed “pitch,” “roll,” and “yaw.” Such position estimation can be useful for various tasks and application. For example, the bearing of a robot relative to a charging station can be useful for allowing the robot to servo to the charging station and recharge its batteries autonomously. The estimation of the distance of a pet from the front door can be used to alert the owner about a possible problem. For indoor environments, it is typically desired to track the (x,y) position of an object in a two-dimensional (2-D) floor plane and its orientation, θ, relative to an axis normal to the floor plane. That is, it can be convenient to assume that a z coordinate of the robot, as well as the robot's roll and pitch angles, are constant. The (x,y) position and the θ orientation of an object are referred to together as the pose of the object.
Numerous devices, processes, sensors, equipment, and mechanisms have been proposed for position estimation. These methods can be divided into two main categories. One category uses beacons in the environment to enable position estimation, and the second category uses natural landmarks in the environment. Because the method and apparatus described herein fall into the first category of beacon-based position estimation or localization, this section will focus on beacon-based localization methods.
Beacons are artificial devices in the environment that can be detected by an appropriate sensing apparatus. Beacons can be passive or active. Examples of passive beacons include retroreflective materials. By projecting a light source onto a retroreflective material, one can create a signature or signal that can be detected readily using one or more appropriate optical sensors. Using the signature or signal, the one or more sensors can determine their positions relative to the beacons and/or relative to the environment.
Active optical beacons emit light that can be detected by an optical sensor. The optical sensor can measure various characteristics of the emitted light, such as the distance to the emitter (using time-of-flight), the bearing to the emitter, the signal strength, and the like. Using such characteristics, one can estimate the position of the sensor using an appropriate technique, such as triangulation or trilateration. These approaches, which use active optical beacons paired with optical sensors, are disadvantageously constrained by line-of-sight between the emitters and the sensors. Without line-of-sight, a sensor will not be able to detect the emitter.